As a rolling bearing which is incorporated into a rotary support portion of various machinery and equipment, for example, a single-row deep groove type ball bearing 1 as shown in FIG. 7 is widely used. The ball bearing 1 is provided with an inner ring 3 having an inner ring raceway 2 on the outer peripheral surface thereof, an outer ring 5 having an outer ring raceway 4 on the inner peripheral surface thereof, a plurality of balls 6 rollably provided between the inner ring raceway 2 and the outer ring raceway 4, and a cage 7 which rollably holds the respective balls 6.
The cage 7 is called a corrugated cage and is made by joining a pair of cage elements 8 and 8 to each other by a plurality of rivets 9 and 9, as shown in FIGS. 8 and 9. Each of the pair of cage elements 8 and 8 is made in the form of a corrugated ring as a whole by performing punching and bending by a press on a material made of a metal plate such as a steel sheet or a stainless steel sheet. The pair of cage elements 8 and 8 respectively has substantially partial spherical shell-shaped (substantially partial spherical) curved plate portions 10 and 10 provided at a plurality of locations in a circumferential direction, flat plate portions 11 and 11 each provided between the curved plate portions 10 and 10 adjacent to each other in the circumferential direction, and through-holes 12 and 12 each provided in the central portion of each of the flat plate portions 11 and 11. Further, each of the rivets 9 and 9 is made of metal such as steel or stainless steel and has a rod portion 13 and a head portion 14 provided at a base end portion of the rod portion 13.
In the cage 7, a caulking portion 15 is formed by crushing a tip portion of each rod portion 13 in a state where the respective flat plate portions 11 and 11 of the pair of cage elements 8 and 8 are superimposed on each other and the rod portion 13 of each of the rivets 9 and 9 is inserted into the through-holes 12 and 12 formed at mutually matching positions of the respective flat plate portions 11 and 11. Then, the respective flat plate portions 11 and 11 superimposed on each other are joined to each other by being pinched by the head portion 14 and the caulking portion 15 of each of the rivets 9 and 9. Then, in this state, portions surrounded by the respective curved plate portions 10 and 10 serve as pockets 16 and 16 for rollably holding the respective balls 6.
There is a case where the ball bearing 1 provided with the cage 7 as described above is incorporated into a rotary support portion or the like of the other end portion of a rotary shaft connected, at one end portion, to a movable scroll configuring a scroll type compressor for refrigerant compression, for example, and used in a situation where the inner ring 3 and the outer ring 5 are eccentric or are inclined. In such a case, in operation, there is a possibility that a significant force may act on the cage 7 from each ball 6 due to a difference or the like in revolution speed between the respective balls 6, and therefore, it is necessary to sufficiently secure the durability of the cage 7. Specifically, it is necessary to sufficiently secure the strength of the pair of cage elements 8 and 8 and the respective rivets 9 and 9 configuring the cage 7. As a method of improving the strength of each of these members 8 and 9, there is a method of increasing the wall thicknesses of the pair of cage elements 8 and 8 or the diameters of the respective rivets 9 and 9. However, due to dimensional constraint, the method cannot be often adopted. In contrast, as a method in which the same object can be achieved almost without involving a change in dimension of each of the members 8 and 9, a method of forming a nitrided layer (a surface-hardened layer by nitriding treatment) on the surface of each of the members 8 and 9 has been known in the past.
In a case of adopting such a method, for example, in a case where nitriding treatment is performed on the pair of cage elements 8 and 8 and the respective rivets 9 and 9 in a state where each of the members is a single body, the nitriding treatment for the respective rivets 9 and 9 becomes troublesome and manufacturing costs increase. That is, each of the rivets 9 and 9 is a small part, and therefore, it is conceivable that the nitriding treatment for the respective rivets 9 and 9 is performed in a state where the respective rivets 9 and 9 are put together in a basket or the like. However, if the nitriding treatment is performed in such a state, it becomes difficult for the nitriding treatment to be performed on a portion where the respective rivets 9 and 9 are in contact with each other. As a result, it becomes difficult to grasp a portion with a nitrided layer formed thereon, among the surfaces of the respective rivets 9 and 9, and there is a possibility that the strength of the respective rivets 9 and 9 may not be stably improved. Therefore, in order to avoid such a disadvantage, it is conceivable that the nitriding treatment for the respective rivets 9 and 9 is performed in a state where the respective rivets 9 and 9 are aligned so as to not come into contact with each other. However, in a case of performing the nitriding treatment in this way, troublesome work for aligning the respective rivets 9 and 9, or a jig or the like for preventing the respective rivets 9 and 9 from falling down is required, and therefore, manufacturing costs increase.
On the other hand, patent document 1 discloses a manufacturing method in which in a state where an intermediate assembly 17 is configured by inserting rod portions 13a and 13a of the respective rivets 9a and 9a into the respective through-holes 12 and 12 of the cage element 8 on one side which configures a corrugated case, as shown in FIG. 10, nitriding treatment is performed on the intermediate assembly 17, and after nitriding treatment is performed on a cage element (not shown) on the other side in a state of being a single body, a pair of cage elements 8 is joined and fixed to each other by the respective rivets 9a and 9a. According to such a manufacturing method, nitriding treatment is performed on the cage element 8 on one side and the respective rivets 9a and 9a at the same time, and therefore, a dedicated nitriding process for the respective rivets 9a and 9a is not required, and thus manufacturing costs are reduced.
However, in the case of the manufacturing method of the related art described above, in a state where the intermediate assembly 17 is configured, a slight gap is present between the outer peripheral surface of each of the rod portions 13a and 13a of the rivets 9a and 9a and the inner peripheral surface of each of the through-holes 12 and 12. That is, the respective rivets 9a and 9a are not subjected to axial falling-out prevention with respect to the respective through-holes 12 and 12. For this reason, there is a possibility that the respective rivets 9a and 9a may fall out from the respective through-holes 12 and 12 during the transport of the intermediate assembly 17 or in a process after the assembling of the intermediate assembly 17.
In order to attain the axial falling-out prevention of the respective rivets 9a and 9a with respect to the respective through-holes 12 and 12 in order to avoid such a disadvantage, it is favorable if each of the rod portions 13a and 13a of the rivets 9a and 9a is press-fitted (internally fitted with an interference fit) into each of the through-holes 12 and 12. That is, in the case of the illustrated structure, each of the rod portions 13a and 13a of the rivets 9a and 9a is composed of a large-diameter portion 18 on the base end side and a small-diameter portion 19 on the tip side. For this reason, it is favorable if the large-diameter portion 18 of these is press-fitted into each of the through-holes 12 and 12 as a press-fitting portion capable of being press-fitted into each of the through-holes 12 and 12. That is, by such a press-fit, the axial falling-out prevention of the respective rivets 9a and 9a with respect to the respective through-holes 12 and 12 is attained.
However, in this case, the following problem arises. That is, usually, each of the inner peripheral surface of each of the through-holes 12 and 12 and the outer peripheral surface of each of the large-diameter portions 18 and 18 of the rivets 9a and 9a is made to be a cylindrical surface. For this reason, in a state after the above-described press-fit, the inner peripheral surface of each of the through-holes 12 and 12 and the outer peripheral surface of each of the large-diameter portions 18 and 18 are in a state of being in close contact with each other (coming into contact with each other without a gap). Therefore, when nitriding treatment is performed on the intermediate assembly 17, nitriding treatment is not performed on the respective peripheral surfaces being in close contact with each other in this manner, and thus a nitrided layer is not formed on each of these peripheral surfaces. In particular, in the case of the illustrated structure, an axial dimension Xa of each of the large-diameter portions 18 and 18 is made larger than an axial dimension Ya of each of the through-holes 12 and 12 (Xa>Ya), and therefore, a state is created in which the entire inner peripheral surface of each of the through-holes 12 and 12 is in close contact with the outer peripheral surface of each of the large-diameter portions 18 and 18. Therefore, a nitrided layer is not formed on the entire inner peripheral surface of each of the through-holes 12 and 12 and a nitrided layer is not formed on the outer peripheral surface of each of the large-diameter portions 18 and 18 by the same axial dimension as that of each of the through-holes 12 and 12. It is desirable that the area of a portion on which a nitrided layer is not formed in this manner is reduced as much as possible from the viewpoint of improving the durability of a corrugated cage after completion.
Further, in the case of the illustrated structure, the axial dimension Xa of each of the large-diameter portions 18 and 18 is made larger than the axial dimension Ya of each of the through-holes 12 and 12 (Xa>Ya). For this reason, as shown in the order of FIGS. 11A and 11B, when the rivet 9a is compressed from both sides in the axial direction between concave portions 21 and 21 of a pair of caulking dies 20 and 20 in order to form the caulking portion 15, a portion of the large-diameter portion 18 is greatly enlarged in diameter on the outside of the respective through-holes 12 and 12. Then, there is a possibility that the flange-shaped portion enlarged in diameter in this manner may protrude into a gap 22 between the inside surfaces of the respective flat plate portions 11 and 11 facing each other (a protruding portion 23 may be formed). If the protruding portion 23 is formed, as shown in FIG. 11B, even after the caulking portion 15 is formed, the gap 22 remains without disappearing, and thus the rigidity of the corrugated case after the completion is lowered, and therefore, it is not preferable.